In recent years, as electronic apparatuses have been miniaturized, demands for high-capacity secondary batteries have been increased. In particular, nonaqueous secondary batteries exhibiting a higher energy density as compared with those of nickel-cadmium cells and nickel metal-hydride batteries have been noted. Heretofore, metals, graphite, and the like have been studied as negative electrode active materials of nonaqueous secondary batteries. However, metal electrodes have problems in that when charge and discharge are repeated, lithium is deposited on electrodes dendritically and, finally, two electrodes are short-circuited. Therefore, carbon materials, in particular, graphite, have been noted because no lithium metal is deposited in a charge and discharge process.
However, there are problems in that since graphite is a flat crystal, graphite tends to be oriented parallel to the current collector in the electrode and, thereby, a high-rate charging/discharging characteristics tends to deteriorate; and since the expansion of the electrode in the charging/discharging cycle is significant, a cycle performances tends to deteriorate.
According to the description in Japanese Unexamined Patent Application Publication No. 10-188959, graphite granulated particles, in which a plurality of graphite particles are bonded in such a way that the oriented faces become not parallel to each other, are used and, thereby, expansion in a thickness direction of the electrode is suppressed and the high-rate charging/discharging characteristics and the cycle performances are improved.
However, the graphite granulated particles described in this patent document have problems in that the performance as a negative electrode material for a lithium ion secondary battery, in particular, a reduction of the irreversible capacity and an improvement of the high-rate charging/discharging characteristics are not always adequate and the cycle performances is also inadequate. The cause of these problems is not certain, but it is believed that the improvement of the electrode orientation ratio of the graphite granulated particles described in the above-described patent document is not always adequate.
According to the description in Japanese Unexamined Patent Application Publication No. 11-263612, natural flake graphite particles are modified into conglobated particles, which are made close to spherical particles and are allowed to have a specific circularity, appearance, and peak intensity ratio, and thereby, good operability is exhibited in slurrying and the applicability and the binding property in preparation of an electrode by application to copper foil or the like are improved.
In this technology described in Japanese Unexamined Patent Application Publication No. 11-263612 as well, the improvement effect thereof is not adequate. The reason for this is believed that the electrode orientation ratio is not improved adequately.